1. Field of the Invention
The present invention pertains to the production of chroma signals from a single image-detecting device. In particular, the present invention relates to the use of dichroic filter stripes to optically color multiplex light from a scene, with the filter stripes oriented perpendicular to the scan lines of an image tube. The image tube then converts the optically multiplexed light signal into a time division multiplexed electronic signal which may be decoded into individual color signals. Standard National Television Standards Committee (NTSC) color signals may be obtained when the optical color multiplexing is achieved by selective use of red and blue dichroic filtering.
2. Description of Prior Art
The production of multiple electronic color signals by a signal television image tube is well known in the art as an advance beyond the use of three image tubes with each image tube producing a single color signal. A basic feature of color television communication is the optical separation of light from the scene to be televised into three distinguishable color light signals. Each light signal comprises information to produce an image of the scene, but only in the particular color frequency range selected for that particular light signal and, therefore, only to the extent that the original light from the scene included light in that frequency range. Usually, the three primary colors, red, blue, and green, are chosen for the three color light signals.
In a three-tube system, each light signal is converted into an electronic signal which, after transmission, is fed to a picture tube to produce a one-color image of the original scene. The three one-color images are optically combined to produce a true color image of the original scene. The system developed by the National Television Standards Committee (NTSC) uses an encoder to convert the three primary color electronic signals into a luminance signal, which encompasses brightness information, and a chrominance signal, which comprises hue and saturation information. The luminance and chrominance signals are transmitted as a composite wave, and decoded at the receiver in terms of the primary colors to produce a true color picture on a single picture tube. To utilize the NTSC transmission system, a single pickup tube system must produce three primary color electronic signals.
Various techniques using striped filter arrangements and dichroic mirrors have been developed for producing three primary color electronic signals from a single television image tube. These techniques take advantage of the fact that a television image tube is a scanning-type detector which temporarily records the image of a scene projected on its photosensitive surface and constructs an electronic signal by methodically scanning the image point by point along a grid composed of parallel scan lines. The resulting electronic signal carries the image information as a function of time. The filter and/or mirror arrangements are used to produce the necessary three light signals in different colors, which are then superimposed on the photosensitive surface of the image tube according to some selected scheme which permits the three images to be distinguished and allows three separate color signals to be retrieved from a single electronic output signal from the image tube. Thus, these single image tube techniques are identifiable according to the scheme by which the three color signals may be distinguished and retrieved.
In each of these single image tube techniques, the striped filter or mirror arrangement acts on the light from the scene to be televised in such a way as to project each of the three color light signals onto the photosensitive surface of the image tube in a periodic pattern. The patterns are usually in the form of stripes oriented at some non-zero angle with respect to the orientation of the image tube scan lines. The stripe patterns of the different color light signals may be mutually parallel or not, depending upon the particular technique employed. Then, in the scanning process, the image tube detects each of the three light signals as a separate, periodic pattern of light intensities, with the three periodic patterns spatially interleaved on the photosensitive surface. The output electronic signal from the image tube thus contains the information from the three separate color light signals interleaved in time; that is, the image tube output signal is a composite of three electronic signals, whose individual wave characteristics, such as amplitude, modulation, and frequency, are determined by the optical intensity, spatial modulation, and period of the corresponding color light pattern on the photosensitive surface of the image tube. When the striped filter arrangement is so constructed that the three color light signals incident on the photosensitive surface of the image tube differ among themselves in period, the information from the three light signals is carried in the output electronic signal from the image tube in three frequency ranges. As a result, three electronic color signals may be recovered from the single output signal from the image tube by appropriate use of low-pass and band-pass filters. Examples of such frequency division multiplex systems may be found in U.S. Pat. Nos. 2,733,291 and 3,530,233. U.S. Pat. Nos. 3,524,014; 3,647,946 and 3,780,212 disclose phase division multiplex systems. In such systems, the three color light signals are projected onto the photosensitive surface of the image tube in such a way that the scanning process detects the three light signals as having the same frequency, but differing in phase. The single output signal from the image tube is then phase demodulated to recover three electronic color signals.
Both the frequency division multiplex and the phase division multiplex systems require very close tolerances in the electronic circuitry and in the image tube in order to avoid undesirable distortion. One source of distortion is imprecision in the scanning which is inherent to some degree in virtually every image pickup tube. Such scanning defects include the scanning beam being out of place at a particular time of the scanning process, and variation in scan velocity. These defects result in distortions in the shape of the image as ultimately produced at the receiver, whether the television system is color or black and white. However, in the case of color systems employing frequency division multiplexing or phase division multiplexing, a variation in the scanning velocity causes an apparent change in the frequencies and phases of the color light signals projected onto the image tube. Consequently, in these color systems, color distortion as well as shape, or geometric, distortion appears in the image at the receiving picture tube. Finally, the band-pass filters employed in these types of color systems are very sensitive to signal frequency, and are therefore additional potential sources of color distortion. It will be appreciated that, while a certain degree of color distortion may be tolerated in many applications, such distortions become prohibitive in some technical applications, particularly where colorimetry is important. It is an advantage, therefore, to employ a color system which does not require extremely close tolerances in the related circuitry, and which effectively eliminates the deterioration of color fidelity caused by imperfect scanning.